Procedures employed commonly in charging pre-foamed synthetic resin particles into a mold cavity consist of firstly metering the necessary volume of pre-foamed synthetic resin particles to be charged, secondly charging the mold cavity with the metered portion of pre-foamed synthetic resin particles through a blow-filling gun, and closing a charging port leading to the mold cavity. The metering of pre-foamed synthetic resin particles to be charged can be achieved by several methods such as, for example, a one-step batch-wise metering of total volume to be charged by using a big metering vessel and a multi-step metering of each divided portion of total volume to be charged by using rather small chambers in the rotor as shown in FIG. 1 and FIG. 2 (see Japanese Patent Publication Nos. 41736/83 and 10296/84, Japanese Laid-Open Patent Application No. 63170/79, British Patent No. 1,445,474 and West German Patent No. 2,363,923).
However, these conventional methods of closing a charging port leading to the mold cavity after the mold cavity has been charged with the entire portion of a metered amount of pre-foamed synthetic resin particles are prone to cause overfilling or underfilling which leads to a high reject rate of foamed molded articles because pre-foamed synthetic resin particles generally have variations in bulk density (ca. 10 to 20% variation with pre-foamed synthetic resin particles).
In the prior art, molding of pre-foamed synthetic resin particles is accomplished with the initial setting of air pressure in the mold cavity being determined in accordance with the bulk density of pre-foamed synthetic resin particles. Therefore, pre-foamed synthetic resin particles having a higher bulk density than the average bulk density are less compressed because of their high compressure stress and they occupy an unduly large space in the mold cavity under compression, with the result that the charging port leading to the mold cavity is closed with the charging in the mold cavity being overfilled up to a cylinder of the blowfilling gun by exceeding the mold cavity. Because of the overfilling at the charging port of the blow-filling gun connected to the mold cavity, the flow of steam among the pre-foamed synthetic resin particles is impeded and the insufficiency of fusion of the pre-foamed synthetic resin particles is liable to yield a foamed molded article with a cleft or a space among the fused foamed synthetic resin particles. On the other hand, pre-foamed synthetic resin particles having a lower bulk density than the average bulk density occupy an unduly small space in the mold cavity under compression, with the result that the charging port leading to the mold cavity is closed with the charging in the mold cavity being shorter than the full capacity of the mold cavity. This has a tendency to cause underfilling at the charging port leading to the mold cavity.